1. Field of the Invention
The present invention relates to the field of toroidal inductive devices, and more particularly to toroidal inductive devices such as transformers, chokes, coils, ballasts, and the like.
2. Description of Related Art
Conventionally available toroidal inductive devices include a toroidal shaped magnetic portion (usually referred to as a core), which is made of strips of grain oriented steel, continuous strips of alloys, or various powdered core arrangements, surrounded by a layer of electrical insulation. An electrical winding is wrapped around the core and distributed along the circumference of the core. This may be done in a toroidal winding machine, for example. Depending upon the type of toroidal inductive device, an additional layer of electrical insulation is wrapped around the electrical winding and a second electrical winding is wound on top of the additional insulation. An outer layer of insulation is typically added on top of the second winding to protect the second winding unless the toroidal device is potted in plastic or the like. A representative toroidal inductive device is described in U.S. Pat. No. 5,838,220.
Toroidal inductive devices provide several key advantages over the more common E-I type inductive devices. For instance, the magnetic core shape minimizes the amount of material required, thereby reducing the overall size and weight of the device. Since the windings are symmetrically spread over the entire magnetic portion of the device, the wire lengths are relatively short, thus further contributing to the reduced size and weight of the device. Additional advantages include less flux leakage, less noise and heat, and in some applications higher reliability.
One significant shortcoming of conventional toroidal inductive devices is that the manufacturing costs far exceed those associated with the more common E-I type inductive devices. The costs are high because complex winding techniques are necessary to wind the electric windings around the toroidal shaped magnetic core.
An additional shortcoming of conventional toroidal inductive devices is that they have a vulnerability to high in-rush current. Such devices generally cannot provide controllable magnetic reluctance, because they are manufactured such that they have no control over a gap in the magnetic flux path. Investigation by the present inventor has revealed that although no gap control is apparent, the flux, which is circular and closed by definition, must pass through an effective gap created by the magnetic portion being spirally constructed and thus not integrally circular. See, for example, FIG. 6, which illustrates magnetic flux 80 in relation to a spiral magnetic member 120. Because the gap is distributed along a length of the magnetic material, the virtual or cumulative gap is very small and thus rendered inconsequential to the operation of the device. The gap is effectively so small that it is necessary in many cases to accommodate the current in-rush problem by adding protective circuitry, such as a current limiting resistor, to the basic device. This increases the overall cost of the device.
An alternative form of toroidal inductive device is known in which the arrangement of the electrical and magnetic portions is basically reversed from the common arrangement described above. In this alternative approach, a magnetic wire is helically wound onto a toroidal electrical winding such that the magnetic portion of the device is formed on the outside of the electrical portion. Such an arrangement is disclosed in International Patent Application Publication No. WO 00/44006. However, this arrangement also requires the use of complex winding techniques and suffers from a lack of magnetic gap control.